Method for preventing flashback in a mixture flowing into a reaction chamber

ABSTRACT

A method for preventing flashback in a reaction chamber that includes providing a mixture of educts flowing through a mixture distribution zone and into the reaction chamber. The mixture distribution zone has an inlet opening and a variable flow cross-section between the inlet opening and the reaction chamber. The method also includes combusting the mixture in the reaction chamber at a combustion rate, and varying the flow cross-section as a function of a volume of the mixture so as to affect a flow rate of the mixture into the reaction chamber such that the flow rate is greater than the combustion rate. In addition, a reactor that includes, an inlet opening for receiving a mixture of educts, a mixture distribution zone disposed downstream of the inlet opening and having a variable flow cross-section, a reaction chamber disposed downstream of the mixture distribution zone, and a regulation device disposed in the mixture distribution zone for varying the flow cross section.

[0001] Priority is claimed to German Patent Application No. DE 102 19 747.4 filed on May 2, 2002, which is incorporated by reference herein.

BACKGROUND

[0002] The present invention relates to a method for preventing flashback in a mixture flowing into a reaction chamber, the flow cross section being varied in a region between an inlet opening and the reaction chamber. Furthermore, the present invention relates to a reactor for carrying out the method specified above, and to the use of the method together with the reactor.

[0003] From the general prior art, reactors are known which are provided with a mixture of educts which are to be converted in a reaction chamber of the reactors. In particular in the case of what are referred to as autothermal reactors which generally have a catalyst in the reaction chamber, the conversion of the educts then takes place in such a way that exothermal and endothermal reactions occur in the reaction chamber. After the start has taken place and/or when the educts are appropriately conditioned there is thus no need for a further supply of thermal energy. The autothermal reformation of a mixture of educts composed of air, steam and a hydrocarbon-containing compound, for example petrol, is an example of such a reaction.

[0004] The educts which flow into the reactor through an inlet opening typically first pass through a zone in which the educts are thoroughly mixed with one another and, if appropriate, individual educts are simultaneously vaporized before they penetrate the actual reaction chamber and are correspondingly converted there. In such reactors, a combustible mixture is then already present in this mixture distribution zone. If there is a flashback from the reaction chamber into the region of the mixture distribution zone or if auto-ignition occurs in this region, the mixture located there is at least partially converted. As a result, thermal energy is released in a region at which it is not required and under certain circumstances has disadvantageous effects, in the form of thermal overloading, on the mixture distribution zone itself and on its immediate surroundings. However, a generally more decisive disadvantage is that the thermal energy released in the mixture distribution zone is lost in the region of the reaction chamber. The conversion of the mixture in the reaction chamber and/or the composition and temperature of the mixture flowing out of the reaction chamber are thus degraded or in the worst case do not occur at all.

[0005] With the stipulation that a hydrogen-containing formate with minimum carbon monoxide content is to be obtained in as far as possible all operating states and load states which occur in the reformation of methanol, German Patent Document DE 195 26 886 C1 proposes a method and a device which is suitable for carrying it out, with which the entire length and/or the effective inlet cross section of an input-end reaction chamber section which is temperature-controlled to a high methanol conversion rate can be set as a function of the throughput rate of mixture to be formed, in such a way that an essentially constant resident time occurs of the gas mixture to be reformed in the reaction chamber section which is temperature-controlled to a high methanol conversion rate. As a result, the methanol reformation can also be carried out with significantly fluctuating throughput rates of gas mixture to be reformed with a constantly high methanol conversion rate and a constantly low formation of undesired carbon monoxide.

[0006] In view of an object of improving the dynamic response behaviour of a reaction of a medium in a reaction chamber which has a catalyst, German Patent Document DE 100 02 025 A1 also discloses a method in which an effective cross section which is accessible to the medium and has the required catalyst is influenced by the pressure of the medium itself. A piston which is arranged in the reactor or in the ducts to the reactor is pressed here against a spring by the medium and, depending on the pressure in the medium opens more or less of the effective cross section.

[0007] By means of the two methods described above or the corresponding devices, the conversion of the methanol or the medium, which according to the statements in the abovementioned German Patent Document DE 100 02 025 A1 will generally be a reformate, can be influenced with a greater or lesser degree of outlay on control, regulation and actuator systems.

[0008] The problems described above relating to flashback, which plays a role especially in autothermal processes, are not recognized in the two abovementioned German patent documents.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a method for avoiding flashback in a mixture flowing into a reaction chamber, and to specify a reactor which can be used to carry out the method.

[0010] The present invention provides a method for preventing flashback in a mixture flowing into a reaction chamber. The flow cross section is varied in a region between an inlet opening and the reaction chamber. The flow cross section is varied as a function of the volume of the inflowing mixture (educts A), in such a way that the flow rate (v) of the mixture (A) is greater for the at least approximately greatest part of the occurring volumes of the inflowing mixture (educts A) is higher than the combustion rate (v_(br)) of the mixture (educts A).

[0011] As a flashback can occur only if the combustion rate v_(br) of the mixture is greater than the flow rate v with which the mixture flows onto the already reacting or burning component which has previously flowed in, a flashback can be avoided by means of a sufficiently high flow rate in the mixture distribution zone.

[0012] The adaptation to a required load state is usually carried out in such reactors by varying the volume flow of the fed-in mixture. If the flow cross sections are configured in such a way that a maximum flow rate v_(max) occurs on the full load, the combustion rate v_(br) can usually be found to be of an order of magnitude of 30 to 50% of the maximum rate as otherwise unnecessarily high pressure losses would be generated in the higher load range, which is important for the operation, as a result of “excessively” small cross sections. However, this then also means that only 50% to 70% of the possible load spread can be used for the reactor as there would be the risk of a flashback with the disadvantages described at the beginning in the remaining 30% to 50%.

[0013] With the method according to the present invention it is then particularly advantageously possible, by varying the flow cross section in the region upstream of the reaction chamber with a predefined volume of flow in accordance with continuity law, to set in each case a flow rate v, which is higher than the combustion rate v_(br). In this way, it is possible to use a much larger part of the load spread than before. By means of the entire range of the load spread of the reactor which can thus be used it is also possible to ensure that no flashback occurs. The release of the thermal energy can thus take place precisely where it is desired, specifically in the reaction chamber itself. The reaction chamber can therefore be used under ideal operating conditions, enabling the desired composition and the desired temperature level to be achieved at the output of the reaction chamber in a reproducible fashion.

[0014] In addition to the problems, already mentioned at the beginning, of the release of thermal energy in an undesired region as a result of the flashback, a disadvantageous formation of byproduct may also occur during the conversion of the educts due to the flashback. These byproducts, namely soot in the case of educts containing carbon, may become deposited and adversely affect the method of operation of the reactor. They accumulate, for example on catalysts, sensors, parts which are to be moved mechanically or the like and impair their functioning. However, preventing the flashback can also avoid this formation of byproducts and the associated disadvantageous consequences.

[0015] According to one very advantageous development of the method according to the present invention, at least in the case of a cold start of a reactor which has the reaction chamber, the flow cross section is increased from the centre of the flow as the volume of the inflowing mixture increases.

[0016] As a result of the fact that the cross section opened from the centre of flow, a central flow into the reaction chamber can be achieved even with a small volume flow. The heat, which is generated centrally in the reaction chamber can be distributed over the surrounding areas of the reaction chamber, in particular if the reaction chamber is constructed, for example, with a catalyst which is located on a carrier material and the heat in the carrier material is conveyed away through thermal conduction. This refinement in the present invention thus makes it possible, on the one hand, to operate continuously in the case of load jumps and, on the other hand, to improve the cold starting behaviour, in particular the more rapid entry into the “normal” operating phase. The reduction in the time necessary for cold starting can be achieved by virtue of the fact that the heat which is conveyed away to the outside by thermal conduction heats up the surrounding areas of the reaction chamber and as a result no, or at least very little, heat is lost, at least in all the load ranges below full load.

[0017] If the reactor is instead in steady-state operating phase without high dynamic requirements and/or in an operating phase in which thermal losses can be accepted without large adverse effects on the quality of generated products, it is possible, according to one very favourable refinement of the present invention, also to vary the flow cross section in such a way that, viewed over a relatively long period of time, all the areas of the reactor are in contact with the inflowing mixture at least for an approximately equal length of time.

[0018] The loading of the reactor, in particular if it has, for example, a catalyst, can thus be compensated over the long-term average so that ageing processes are distributed uniformly over the reactor. The service life of the reactor can thus be increased.

[0019] A reactor for carrying out the method according to the present invention has, one following the other in the direction of flow, an inlet opening for the educts, a mixture distribution zone and a reaction zone, regulation devices for changing the flow cross section being arranged in the mixture distribution zone.

[0020] The method according to the present invention can be implemented in an ideal way with such an embodiment of the reactor. Here, it is virtually insignificant how the regulation devices for changing the cross section are embodied as long as they reliably fulfil the required function under the conditions prevailing in the mixture distribution zone, for example high temperature, aggressive educts etc. The regulation devices could be embodied as continuously acting regulation devices, for example, in the manner of iris diaphragms such as are known, for example, from optics.

[0021] As an alternative to this, the regulation devices for changing the cross section in the mixture distribution zone are embodied, according to one very favourable element of the reactor, in such a way that the mixture distribution zone is divided into a plurality of segments, it being possible to at least partially close off inflow openings in at least some of the segments.

[0022] This design which can be implemented in a very robust way permits a good method of operation of the regulation device which has a high degree of immunity to faults, even under unfavourable conditions, for example in terms of the temperature and the aggressiveness of the media. Furthermore, selective influencing of the flow which is achieved in the mixture distribution zone by means of corresponding built-in elements, diffusers or the like, can be maintained, or achieved at all in the first place, even with relatively low volume flows by means of the segmentation. If desired, the effects mentioned above can also be adapted to the respectively predefined volume flows by means of the refinement of the geometry of the segments for said volume flows. The inflow can thus be appropriately optimized not only in terms of the flow rate but also in terms of the formation of the flow. In particular, it is possible here to avoid dead zones in the flow in which there is not a sufficiently high flow rate of the flow. As a result, on the one hand, the undesired conversion of the educts and, on the other hand, deposition of byproducts which are formed can be avoided in these dead zones in a particularly advantageous way.

[0023] One particularly advantageous use of the method according to the present invention and of the abovementioned reactor is the autothermal reformation of an educt mixture, containing at least oxygen, water, in particular steam, and a hydrocarbon-containing compound, preferably petrol or diesel, in order to generate a hydrogen-containing gas for operating a fuel cell, in particular the fuel cell of an auxiliary power unit.

[0024] In particular for such a use in a gas generating system for a fuel cell, the abovementioned advantages can be of particular advantage in terms of achieving the largest possible load spread and a robust and reliably operating design. If the fuel cell is used in a mobile system, for example a vehicle or the like, the advantages already mentioned are particularly advantageous with respect to the basic requirements made of the vehicle components in terms of robustness, complexity, weight and dynamic method of operation.

[0025] The refinement of the method according to the present invention in which, at least in the case of a cold start of a reactor which has the reaction chamber, the flow cross section is increased from the centre of the flow as the volume of the inflowing mixture increases can also be used very favourably in this specific case of application in mobile systems as, in such systems, cold starts occur very frequently and accordingly an improvement in the cold start behaviour constitutes a decisive improvement in the entire system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further advantageous refinements of the present invention emerge from the claims and from the exemplary embodiment which is illustrated below with reference to the drawing, in which:

[0027]FIG. 1 shows a reactor which is operated as an autothermal reformer as well as possible temperature profiles T over its overall length x;

[0028]FIG. 2 shows a diagram of a flow rate v in the region of the inflow into a reactor chamber as a function of a load L;

[0029]FIG. 3 shows a possible refinement of the reactor with regulation devices for changing the flow cross section;

[0030]FIG. 4 shows an alternative refinement of the reactor with regulation devices for changing the flow cross section;

[0031]FIG. 5 shows a sectional view along the line V-V in the embodiment in FIG. 4;

[0032]FIG. 6 shows a further alternative refinment of the reactor with regulation devices for changing the flow cross section; and

[0033]FIG. 7 shows a sectional view along the line VII-VII in the embodiment in FIG. 6.

DETAILED DESCRIPTION

[0034]FIG. 1 illustrates a reactor 1 which is intended to be operated here as an autothermal reformer, the present invention being merely explained by means of this example and there is no intention to restrict it to the specific application case of the autothermal reformer.

[0035] According to its embodiment as an autothermal reformer for generating a hydrogen-containing gas, the reactor 1 has a reaction chamber 2 which contains, on a carrier structure 3, a catalytically active material—which are referred to below as a catalystic carrier 3. Educts A, for example air, hydrogen and petrol or diesel, flowing into the reactor 1 pass through an inlet opening 4 into a mixture distribution zone 5 in which, if necessary, they are thoroughly mixed and components which are still possibly present in fluid form are vaporized and, if appropriate, are superheated. Furthermore, the educts A are distributed by the mixture distribution zone 5, for example by means of one or more diffusers, in such a way that they flow into the reaction chamber 2 as uniformly and homogeneously as possible.

[0036] The reaction chamber 2 which is provided with the catalyst carrier 3 can be divided into two different zones: one exothermal reaction zone 6 through which educts coming from the mixture distribution zone 5 firstly flow, and an endothermal reaction zone 7 which follows the latter in the direction of flow. Furthermore, the mixture distribution zone 5 has regulation devices 8 which are indicated here in principle by a dot-dashed line and about which more details will be given later.

[0037] In the view in FIG. 1, a number of temperature profiles T are additionally plotted against an overall length x of the reactor 1. The constant temperature profile T_(min) indicates the temperature which the reformate obtained from the educts A must at least have when it exits the reactor chamber 2. This temperature T_(min) is determined by the following components, for example gas purification devices, shift stages or the like. The temperature profile T₁ would be ideal for reaching this temperature T_(min) at the outlet of the reactor chamber 2 with the best possible level of efficiency and thus the lowest possible inlet temperature T₁. In the case of the temperature profile T₁ leaving the inlet temperature T₁, the educts A release the thermal energy Q₁ contained in them during the reaction in the exothermal reaction zone 6. The volume flow, which then cools in the region of the endothermal reaction zone 7, then reaches, at the output of the endothermal reactor zone 7 and thus of the reacton chamber 2, the temperature T_(1a) which is higher than or equal to the temperature T_(min).

[0038] However, as a combustible mixture of the educts A is then already present in the mixture distribution zone 5, it is possible, as already explained at the beginning, for a flashback which is initiated by the hot catalyst carrier 3 to occur from the reaction chamber 2 into the region of the mixture distribution zone 5, which brings about an at least partial conversion of the educts A, combined with a release of thermal energy. The temperature profile T₂ will then typically occur.

[0039] The temperature profile T₂ starts at the same inlet temperature T₁. However, a release of the thermal energy content Q₂ which is contained in the educts A, and corresponds in its absolute value to Q₁, will then already occur in the region of the mixture distribution zone 5. However, as a result of this premature release of the energy Q₂ said energy Q₂ is absent from the region of the reaction chamber 2. The resulting outlet temperature T₂a of the volume flow out of the reaction chamber 2 is therefore lower than the required temperature T_(min). In addition to this, there is generally also worsening of the conversion of the educts A used so that in the following components it is necessary to make greater expenditure in order to purify the reformate.

[0040] So that, nevertheless, a sufficiently high outlet temperature can then still be achieved, the temperature profile T₂ can be shifted upwards, towards the higher temperatures. However, the resulting temperature profile T₃ requires a higher inlet temperature T₁, and thus reduces the efficiency of the reactor 1.

[0041]FIG. 2 is a diagram illustrating the dependence between a flow rate v in the region of the inflow of the reaction chamber 2 and a load L which represents the required conversion of material taking place or the volume flow of the educts A. Both the flow rate v and the load L are standardized to the respectively occurring values of the maximum flow rate v_(max) and of the full load L_(max) and given in percentages.

[0042] The abovementioned undesired release of energy as a result of the flashback or, under certain circumstances, also as a result of auto-ignition of the educts A, will, as already mentioned at the beginning, occur only if the flow rate v of the educts A is lower than the combustion rate v_(br). In the diagram in FIG. 2, the combustion rate v_(br) is then set at 40% of the maximum flow rate v_(max). The relationship between the flow rate v and load L is given by the dashed curve 9. From its point of intersection 10 with the constant V_(br) it is possible to read off that an operating mode of the reactor 1 which is optimized and reliable in terms of efficiency is possible only with a load spread between 40% and 100% of the full load L_(max).

[0043] In order to reduce the problems of flashback and to be able to use the greater part of the load spread accompanied by optimized efficiency, that is to say with a temperature profile which is analogous to T₁, the regulation devices 8 are provided in the mixture distribution zone 5 of the reactor 1. These regulation devices 8 are used to vary the flow cross section in the region of the mixture distribution zone 5 as a function of the volume flow of the inflowing educts A so that variable flow rates v can be set in accordance with the continuity law. As a result, the flow rate v in the region before inlet into the reaction chamber 2, and here in particular in the region between the inlet opening 4 and the reaction chamber 2, can be set, as a function of the volume flow of the educts A, which can either be measured or originate in an ideal fashion from the predefined values for the metering of the educts A, in such a way that said flow rate v is greater than the combustion rate v_(br) over the greatest possible area of the load spread. Here, all of the predefined values which are as characteristic as possible of the conversion, or only some of them, can be used for the metering, for example the metered quantity of fuel. The adaptation of the flow rate v is very important in precisely this region of the mixture distribution zone 5 as here usually a widening of the flow cross section is provided in order to distribute the educts A, at least in the case of the full load, over the entire cross section of the reaction chamber 2. The problems relating to flashback are therefore concentrated essentially in this region upstream of the reaction chamber 2.

[0044]FIG. 3 shows, in a sectional view of half the rotationally symmetrical structure of a reactor 1, a possible refinement of the regulation devices 8 in the region of the mixture distribution zone 5 of the reactor 1. The regulation devices 8 are composed here of a plurality of annular walls which divide the mixture distribution zone 5 into segments 11. The segments 11, which form annular ducts 111 here, can be closed by means of annular covering elements 13 which correspond to the inlet cross sections 12 of said segments 11. The flow cross section in the mixture distribution zone 5 can thus be released or blocked in a plurality of stages. Of course, at least two such segments 11 are necessary to ensure the desired method of operation. The maximum number is determined by the structural space and the cross section through which there is a flow in the mixture distribution zone 5, as well as by the load spread.

[0045] For the present application case of the exemplary embodiment, a number of five annular ducts 11 with correspondingly four of the annular coverings 13 have been selected. The resulting profile of the flow rate v given successive opening of the individual annular ducts 111 as the load L rises is illustrated by means of the dot-dashed curve 14 in FIG. 2. After approximately 8% of the full load L_(max) has been reached, all the flow rates v are above the combustion rate v_(br). The area of the load spread which can be used under approximately ideal operating conditions is therefore between 8% and 100%. This constitutes a significant improvement over the design described by means of the curve 9.

[0046] The design of the regulation devices 8 in FIG. 3 shows that here a distribution of the inflowing mixture of the educts A is to be reached in the mixture distribution zone 5. For this purpose, for the reasons already mentioned above, the flow cross section widens in the direction of the catalyst carrier 3 in the manner of a diffuser, by virtue of the use of a flow distributor 15. The structure of the segments 11 is then selected which is such that each of the inlet cross sections 12 has a specific portion of the sum of the inlet cross sections 12, and thus of the available flow cross section. Each of the segments 11 also has an outlet cross section 16 which has the same proportion of the sum of the outlet cross sections 16 as its inlet cross section 12 had of the sum of the inlet cross sections 12. The fluidic effect generated by the widening of the flow cross section that is to say the diffuser, is thus transferred to each individual segment 11 so that a comparable flow onto the catalyst carrier 3 is always achieved irrespective of the volume flow of the educts A and the number of the closed or opened segments 11.

[0047] According to the exemplary embodiment present here, the annular covering elements 13 are arranged fixedly with respect to one another on a common carrier 17. In this design which is very robust and also has a high level of immunity to faults even under the possibly aggressive conditions in the mixture distribution zone 5, the annular covering elements 13 are arranged on the carrier 17 in such a way that they can each be displaced together and in the process successively open the individual annular ducts 111 in the manner predefined by the arrangement, or successively enlarge the flow cross section in the direction of the individual ducts 111. Instead of the theoretically also conceivable displacement of all the covering elements 13, in each case individually and independent of one another, the common carrier 17 results in a very robust design. The carrier 17 itself is displaced in the direction of the main flow of the educts A. This is significantly more favourable in terms of the soiling of sliding faces in comparison with a displacement transversely with respect to said flow. Furthermore, the parts which are to be displaced with respect to one another cannot be pressed onto one another by the flow pressure, which would strongly increase the friction and thus the force necessary for activation. The driver of the carrier 17 can be displaced very slightly towards the outside of the reactor 1 if the carrier 17 is constructed to have corresponding length or if there is a suitable transmission element, for example a push and pull rod. The situation is comparable for guides and seals. The design can thus be implemented independently of the conditions in terms of temperature and aggressiveness of the educts A prevailing in the region of the inlet opening 4 and in the region of the mixture distribution zone 5, so that the control and/or regulation as well as the sealing and guidance can be carried out with an appropriately high level of reliability but yet easily and cost-effectively.

[0048] The opening and the closing of the individual segments 11 is carried out by means of the arrangements of the annular covering elements 13 in such a way that segments 11 which are adjacent to one another are opened or closed successively. This has the result that the areas into which there is a new inflow in the region of catalyst carrier 3 or the areas in which there is no longer an inflow in each case lie directly next to one another. They can thus interact and very easily exchange thermal energy with one another so that the operation of the reactor 1 becomes more homogeneous and is thus improved with respect to the desired conversion.

[0049] In particular in the case of a cold start of the reactor 1, this can be used very favourably as there is firstly an inflow onto the catalyst carrier 3 in a centrally located region 18 through successive opening of the individual segments 11 from the inside to the outside. As a result of the thermal conduction occuring in all directions in the catalyst carrier 3, heat passes first from this first-used central region 18 into all the surrounding regions. If the inflow into the surrounding regions is then released through an increasing volume flow of the educts A by opening the adjacent segments 11, said regions are already pre-heated so that the catalytically active material very quickly reaches its operating temperature or has possibly already reached it. The conversion of the educts A starts up very quickly, and the time required to cold start the reactor 1 can be reduced. In addition, under all conditions of partial load the conveying of heat out of the region of the reactor chamber 2 into the surroundings, which always constitutes a heat loss, is avoided or at least significantly reduced, and the effectiveness of the reactor 1 thus increased.

[0050]FIG. 4 illustrates an alternative embodiment of the regulation devices 8. Here too, the mixture distribution zone 5 is divided into individual segments 11. These are embodied as concentric annular ducts 11 1 about a central duct 112 which is arranged in the central region. The segments 11 whose cross section is illustrated once more at the junction between the inlet opening 4 and the mixture distribution zone 5 in FIG. 4, in the region of the inlet opening 4, are also embodied here as regions which open in the direction of the flow. The requirements which have already been mentioned above and preferred refinements apply here correspondingly with the exception of those of the annular covering elements 13. The reference symbols in FIG. 4 and the following figures of the exemplary embodiment are used analogously to those in FIG. 3 when there is a comparable method of operation of the components and/or cross sections.

[0051] Instead of the annular covering elements 13, the regulation devices 8 according to the refinement according to FIG. 4 have sheaths 19 and a needle 20. These are shown in FIG. 5, which is a sectional view along the line V-V in FIG. 4 indicated in the abovementioned cross section. The abovementioned functional principle of the regulation devices does not change here. However, as a result of the configuration with the sheaths 19 and the needle 20, each individual segment 11 or its inlet cross section 12 can be selective if opened or closed. The needle 20 and the sheaths 19 are moved for this purpose in the direction of flow of the educts A. The drive can also be displaced very easily towards the outside of the reactor 1 here if needles 20 and/or sheaths 19 are given a corresponding length. The situation is comparable for guides and seals. The design can also therefore be implemented here independently of the conditions in terms of temperature and aggressiveness of the educts A prevailing in the region of the inlet opening 4 and in the region of the mixture distribution zone 5.

[0052] In order to ensure an exchange of the educts A over the entire range of the inlet opening 4, and thus to provide the possibility of being able to open and close the segments 11 in any desired sequence, at least the sheaths 19, which are arranged between the needle 20 and the sheath which is arranged furthest away from the needle, should have openings 21. By means of these openings 21, which may be embodied as drilled holes, windows or the like and may, under certain circumstances, also constitute the approximately largest part of the sheath 19, it is possible to ensure the exchange of the educts A over the entire cross section of the inlet opening 4. If the needle 20 is guided backwards out of the structure, it may, under certain circumstances, also be appropriate or necessary here, as illustrated, if the outermost of the sheaths 19 also has openings 21 so that, for example, use is made possible with a volume flow to the inlet opening 4 at a right angle with respect to the needle 20.

[0053] The operating strategy during the opening and/or closing of the individual segments 11 can thus be freely adapted to the requirements of the reactor 1, and here in particular to those of the reaction chamber 2 or of the catalyst carrier 3, without the need to take into account mechanical specifications due to the design of the regulation devices 8. The use of operation strategies which have already been mentioned at the beginning for optimizing the cold start behaviour, for optimizing the ageing processes etc. thus becomes possible in a very easy and flexible way.

[0054] Furthermore, by means of a design such as described in FIGS. 4 and 5 it is possible to avoid dead zones of the flow of the educts in the region of the mixture distribution zone 5 or at least reduce them. The formation of byproducts, for example soot during the autothermal reformation of petrol or in particular of diesel, as already explained in the beginning, can thus be prevented in an ideal way. Soiling of the mechanism, especially coating of the catalytically active material with the soot, is thus prevented. It is therefore possible to increase the operational reliability of the reactor 1, as well as its service life and the quality of the reformate.

[0055]FIG. 6 illustrates a further alternative embodiment of the regulation devices 8. Here too, the mixture distribution zone 5 is divided into individual segments 11. These segments 11 are formed by dividing walls which segment the mixture distribution zone 5 which has a circular cross-sectional shape here into three line regions 113 in the shape of a third of a circle. The segments 11 whose cross section is illustrated once more at a junction between the inlet opening 4 and the mixture distribution zone 5 in FIG. 6 in the region of the inlet opening 4 are also embodied here as regions which open in the direction of flow. In the region between the inlet opening 4 and the mixture distribution zone 5, each of the line regions 113 has an inlet opening 22. These inlet openings 22, which correspond in their function approximately to the abovementioned inlet cross sections 12, can in turn be closed so that the individual line regions 113 can be opened and closed individually and independently of one another.

[0056] In theory, any desired method for opening or closing is possible, but it is particularly favourable to adopt the solution illustrated schematically in FIG. 7, in which the inflow openings 22 are each closed and/or opened by means of needles 23. The method of operation and the bearing/guidance as well as the driving of the needles 23 are the same as has already been described above with respect to the needle 20 and the sheaths 19.

[0057] All the embodiments of the regulation devices cover, in the reactor 1, the favourable possibilities mentioned above and in particular discussed in general within the scope of FIG. 3. Furthermore, all the conceivable and appropriate combinations of individual features from the various exemplary embodiments to form further regulation devices 8 are conceivable. These also correspondingly permit favourable methods of functioning and operating for the reactor 1, and fall within the scope of the present invention. 

What is claimed is:
 1. A method for preventing flashback in a reaction chamber, the method comprising: providing a mixture of educts flowing through a mixture distribution zone and into the reaction chamber, the mixture distribution zone having an inlet opening and a variable flow cross-section between the inlet opening and the reaction chamber; combusting the mixture in the reaction chamber at a combustion rate; and varying the flow cross-section as a function of a volume of the mixture so as to affect a flow rate of the mixture into the reaction chamber such that the flow rate is greater than the combustion rate.
 2. The method as recited in claim 1, wherein the mixture distribution zone includes a plurality of flow segments and the varying of the flow cross-section includes opening or closing at least one of the plurality of flow segments.
 3. The method as recited in claim 2, wherein the varying of the flow cross-section includes increasing the flow cross-section by opening at least one adjacent flow segment.
 4. The method as recited in claim 2 wherein the varying of the flow cross-section includes decreasing the flow cross-section by closing at least one adjacent flow segment.
 5. The method as recited in claim 1, wherein the mixture includes a plurality of components and the varying of the flow cross section is further performed as a function of a predefined value for metering at least one component of the mixture.
 6. The method as recited in claim 1, wherein the varying of the flow cross-section includes increasing the flow cross-section from a center region of the flow as the volume increases.
 7. The method as recited in claim 2, wherein the varying includes opening or closing each of the plurality of flow segments for a predetermined time period such that each of the plurality of flow segments is in contact with the mixture for approximately a same length of time.
 8. A reactor comprising: an inlet opening for receiving a mixture of educts; a mixture distribution zone disposed downstream of the inlet opening and having a variable flow cross-section; a reaction chamber disposed downstream of the mixture distribution zone; and a regulation device disposed in the mixture distribution zone for varying the flow cross section.
 9. The reactor as recited in claim 8, wherein the regulation device includes dividers for dividing the mixture distribution zone into a plurality of segments each having inflow openings, and wherein at least one of the inflow openings is capable of being at least partially closed.
 10. The reactor as recited in claim 9, wherein the each of the plurality of segments includes an annular duct.
 11. The reactor as recited in claim 10, further comprising a first annular covering element corresponding to a respective one of the plurality of annular ducts.
 12. The reactor as recited in claim 11, wherein the first annular covering element is moveable to a closed position in a direction of flow through the mixture distribution zone.
 13. The reactor as recited in claim 11, further comprising a second annular covering element fixedly connected to the first annular covering element and moveable to a closed position together with the first annular covering element.
 14. The reactor as recited in claim 9, further comprising at least one sheath for at least partially closing the at least one segment.
 15. The reactor as recited in claim 10, further comprising a needle and a circular center duct disposed radially inward of the annular ducts, the circular center duct being at least partially closeable by the needle.
 16. The reactor as recited in claim 9, further comprising a plurality of needles, each of the plurality of needles moveable to close one of the plurality of inflow openings.
 17. The reactor as recited in claim 9, wherein at least one of the plurality of segments includes a widening flow path along a length of the segment in a direction of flow.
 18. The reactor as recited in claim 9, wherein each of the segments further includes an outlet and wherein a ratio of each inflow cross-section to a sum of the plurality of inflow cross-sections corresponds to a ratio of a respective outlet cross section to a sum of the plurality of outlet cross sections.
 19. The reactor as recited in claim 8, wherein the reaction chamber includes a catalytically active material disposed on a carrier structure.
 20. The reactor as recited in claim 8, wherein the educts include at least oxygen, water, and a hydrocarbon-containing compound for generating a hydrogen containing gas.
 21. The reactor as recited in claim 20, wherein the water is in the form of steam.
 22. The reactor as recited in claim 20, wherein the hydrocarbon-containing compound includes at least one of diesel or gasoline.
 23. The reactor as recited in claim 20, further comprising a fuel cell operated using the hydrogen containing gas
 24. The reactor as recited in claim 23, wherein the fuel cell includes a fuel cell of an auxiliary power unit. 